1. FIELD OF THE INVENTION
The present invention relates to an apparatus and method for purifying water and, more particularly, to an apparatus and method for producing ultrapure water used as rinsing water for VLSIs and the like.
2. DESCRIPTION OF THE RELATED ART
A large quantity of pure water or ultrapure water is employed in the electronic industry. As the degree of integration of LSIs increases, there is an ever increasing demand for high purity of ultrapure water. An ion exchange unit, an ultraviolet sterilizer, a cartridge filter, a reverse osmosis unit (hereinafter referred to as an RO), an ultrafilter membrane, and other components have been heretofore combined together in a multiple-stage structure so as to produce ultrapure water.
FIG. 4 is a flowchart showing steps in which water is treated by a conventional water purifying apparatus. Raw water is received in a raw water tank 1, and then suspended solids in the raw water are removed by a coagulation filter 2. A coagulant such as polyaluminum chloride is added to the raw water. The raw water treated by the coagulation filter 2 is then stored in a filtered water tank 3. Thereafter, a so-called two-bed three-tower type ion exchange unit, composed of a cation exchange tower 4, a decarbonating tower 5, and an anion exchange tower 6, removes ions from the filtered water. The cation exchange tower 4 as well as the anion exchange tower 6 also removes fine particles and organic substances from the water. The water treated by the anion exchange tower 6 is stored temporarily in a treated water tank 7, and then passed through a heat exchanger 8, an ultraviolet sterilizer 9, and a safety filter 10 before it is fed to a RO 11. The water is treated by the RO 11 and is stored in an RO treated water tank 12. A mixed bed ion exchange unit 13 then removes residual ions in the water, thus rendering the resistivity of water to 10M.OMEGA..multidot.cm or more. The water is supplied to a point of use or transferred to a polishing system to produce ultrapure water.
The conventional water purifying apparatus is configured as described above. The RO 11 is a membrane having pore size of several angstrom, and removes ions, fine particles, fungi, high-molecular organic substances, and colloidal substances such as colloidal silica, all of which are in water. The RO 11 is made of a thin membrane and is operated under a high pressure of 10-30 Kg/cm.sup.2. Therefore, some of the above substances pass through the RO 11, thus reducing the removal ratio of colloidal substances in particular.
Recently, colloidal substances, such as colloidal silica, colloidal metal, and colloidal organic substances, have become the most serious problem when producing high purity rinsing water for VLSIs and ULSIs. The thus-constructed conventional water purifying apparatus has difficulty in removing the colloidal substances in water.